Among various types of vehicles, such as automobiles, is one employing a vehicle body structure where a sub frame having suspensions and an engine supported thereon is mounted to a vehicle body frame. Examples of the structure for mounting the sub frame to the vehicle body frame include a fixing-type mounting structure and a floating-type mounting structure (floating support structure). The fixing-type mounting structure is a structure that mounts the sub frame directly to the vehicle body frame. Where the fixing-type mounting structure is employed, the sub frame can absorb collision energy together with the vehicle body frame when the collision energy has acted on the vehicle body from the front of the vehicle.
During travel of a vehicle, not only vibrations are transmitted from wheels (i.e., road wheels) to the sub frame via suspensions, but also vibrations are transmitted from an engine to the sub frame via an engine mount. With the fixing-type mounting structure, however, these vibrations would be transmitted from the sub frame to the vehicle body frame. Therefore, the fixing-type mounting structure is disadvantageous in that it can not effectively restrain vibrations and sound noise from being transmitted to the interior of a vehicle compartment to achieve an enhanced riding comfort and amenity of vehicle occupants.
The floating-type mounting structure is a structure for mounting the sub frame to the vehicle body frame via elastic members (anti-vibration rubber). Where the floating-type mounting structure is employed, vibrations transmitted from the road wheels to the sub frame via the suspensions and vibrations transmitted from the engine to the sub frame via the engine amount during travel of the vehicle can be lessened by the elastic members so that the vibrations are effectively prevented from being transmitted to the vehicle compartment. Therefore, the floating-type mounting structure is advantageous in that it can restrain vibrations and sound noise from being transmitted to the interior of the vehicle compartment to achieve an enhanced riding comfort and amenity of vehicle occupants. However, because the elastic members are interposed between the vehicle body frame and the sub frame, the elastic members would be deformed when collision energy has acted on the vehicle body from the front of the vehicle. Thus, some measures have to be taken for efficiently absorbing the collision energy by means of the sub frame. In such a case, it is necessary not only to avoid the structure from becoming complicated but also to minimize increase of the weight of the vehicle; increase of the weight of the vehicle would become a cause of lowering the mileage (i.e., a distance which the vehicle can travel per unit quantity of fuel).
In recent years, development of an improved floating-type mounting structure with collision energy into consideration is under way. The improved floating-type mounting structure is constructed in such a manner that, when collision energy more than a predetermined level has acted on the vehicle body frame, some of a plurality of bolts connecting the sub frame to the vehicle body frame are broken to cause the sub frame to fall from the vehicle body frame so as to lessen an impact on the vehicle compartment. One example of such an improved floating-type mounting structure is disclosed in Japanese Patent Publication No. 4026815. With the improved floating-type mounting structure too, it is more preferable that collision energy be effectively absorbed by the sub frame.
As apparent from the foregoing, the vehicle body structures are requested to satisfy, at the time of a collision of the vehicle, both (1) the requirement of restraining vibrations and sound noise from being transmitted to the interior of the vehicle compartment to thereby achieve an enhanced riding comfort and amenity of vehicle occupants and (2) the requirement of efficiently absorbing collision energy by means of the vehicle body frame and sub frame.